Advances to the next document in the set and returns the doc it is
currently on, or DocIdSetIterator.NO_MORE_DOCS if there are no more docs in the
set.NOTE: after the iterator has exhausted you should not call this
method, as it may result in unpredicted behavior.

advance

Advances to the first beyond the current whose document number is greater
than or equal to target, and returns the document number itself.
Exhausts the iterator and returns DocIdSetIterator.NO_MORE_DOCS if target
is greater than the highest document number in the set.

The behavior of this method is undefined when called with
target ≤ current, or after the iterator has exhausted.
Both cases may result in unpredicted behavior.

NOTE: this method may be called with DocIdSetIterator.NO_MORE_DOCS for
efficiency by some Scorers. If your implementation cannot efficiently
determine that it should exhaust, it is recommended that you check for that
value in each call to this method.

nextDoc

Advances to the next document in the set and returns the doc it is
currently on, or DocIdSetIterator.NO_MORE_DOCS if there are no more docs in the
set.NOTE: after the iterator has exhausted you should not call this
method, as it may result in unpredicted behavior.

nextPosition

Returns the next position. You should only call this
up to DocsEnum.freq() times else
the behavior is not defined. If positions were not
indexed this will return -1; this only happens if
offsets were indexed and you passed needsOffset=true
when pulling the enum.